In a conventional drive train of a motor vehicle a planetary automatic transmission is usually used in combination with a hydrodynamic torque converter. Via the torque converter the driveshaft of the internal combustion engine is in driving connection with the input shaft of the automatic transmission, such that by virtue of the hydraulic-elastic connection of the driveshaft of the internal combustion engine to the input shaft of the automatic transmission, both rotational vibrations of the driveshaft of the internal combustion engine and also shift-originating load pulses of the automatic transmission are damped and therefore transmitted only in a very weakened form to the respective other assembly, i.e. into the automatic transmission or into the internal combustion engine.
A further advantage of a torque converter is that when the vehicle is at rest or driving or rolling at low speed, the torque is amplified by a relatively large factor up to the value 2.5. Thanks to the resulting high starting torque, motor vehicles of this type have high starting acceleration and high climbing capacity from rest.
Furthermore, when the internal combustion engine is idling and a gear is engaged, a crawling torque is available by virtue of which, when starting on a hill, the motor vehicle is prevented from rolling backward when the driver changes from the brake pedal to the accelerator pedal, and thanks to which maneuvering on flat ground can be controlled by a harder or less hard actuation of the brake pedal alone.
A disadvantage of a torque converter, however, is that compared with a manual or automated shift transmission the fuel consumption is higher, as a result of the drive power required for an associated oil pump, the transmission slip within the torque converter and the higher idling power of the internal combustion engine required for the crawling torque.
In a drive train of a heavy utility vehicle, such as a heavy-load tractor or a self-propelling crane vehicle, a built-in manual gearshift transmission is usually connected to the internal combustion engine by a so-termed converter shifting clutch. A converter shifting clutch is an arrangement in series comprising a hydrodynamic torque converter and a controllable separator clutch, i.e. one that can be automatically engaged and disengaged. The torque converter, usually provided with a bridging clutch, serves when the bridging clutch is disengaged, the separator clutch is engaged and a starting gear is engaged, to enable jerk-free and wear-free starting. During this, when there is a large speed difference between the pump impeller and the turbine wheel of the torque converter, which occurs when the vehicle is at rest or moving at low driving or rolling speeds, a relatively large torque increase by a factor of up to 2.5 acts on the input shaft of the manual gearshift transmission. Thanks to the resultant high starting torque, motor vehicles of this type have comparatively high starting acceleration and high climbing capacity from rest. Furthermore, the crawling torque acting on the output shaft of the manual gearshift transmission in this operating condition prevents rolling backward when the driver changes from the brake pedal to the accelerator pedal while starting on a hill, whereas maneuvering on flat ground can be controlled by a harder or less hard actuation of the brake pedal alone. The separator clutch is now disengaged to allow load-free shifting of the gears of the manual gearshift transmission. At higher speeds and when higher gears are engaged the bridging clutch is usually engaged. However, to damp shift-originated load pulses by the torque converter it can be temporarily disengaged even when shifting between higher gears. The disadvantages of a converter, however, are the higher fuel consumption that results from the drive power for an associated oil pump of the torque converter, the transmission slip inside the torque converter when the bridging clutch is disengaged, and the higher idling power of the internal combustion engine required for producing the crawling torque.
If from conventional drive trains of that type a hybrid drive train is to be derived, then an electric machine that can be operated as a motor and generator can be provided instead of the hydraulic torque converter or a converter bridging clutch, so that the electric machine takes over the function of the torque converter at least in part. For the hybrid drive train so constituted to keep to the dimensions of a conventional drive train and thus to be able to be fitted in a motor vehicle in place of the latter without problems, the electric machine comprising a stator and a rotor is expediently arranged coaxially over the input shaft of the transmission, with the rotor of the electric machine in driving connection with the input shaft.
To compensate for a rotational speed difference between the driveshaft of the internal combustion engine and the input shaft of the transmission that exists when the vehicle is starting from rest, the driveshaft is connected to the input shaft by means of a controllable separator clutch, i.e. one that can be automatically engaged and disengaged, this separator clutch, however, in contrast to a converter shifting clutch, being arranged between the internal combustion engine and the input shaft of the manual gearshift transmission in driving connection with the electric machine.
During driving operation the electric machine can optionally be switched in without force, or used as a generator for charging an electrical energy accumulator, or used as an electric motor for driving the motor vehicle. During driving operation powered by the internal combustion engine, if the separator clutch is engaged, particularly when accelerating rapidly and driving up a steep uphill stretch, the electric machine can be used to support the internal combustion engine in so-termed ‘boost’ operation, whereas if the separator clutch is disengaged, particularly when starting off and when driving in urban areas with emission restrictions, it can be used on its own as the drive motor in purely electric driving operation. When the transmission is in neutral, the electric machine in combination with engaging the separator clutch can also be used for starting and for actively stopping the internal combustion engine.
Such a hybrid drive train with a transmission preferably in the form of a planetary automatic transmission is described, for example, in two versions in DE 199 17 665 A1. In the first embodiment variant illustrated by FIG. 1 in that document the driveshaft of the internal combustion engine can be connected by means of a controllable clutch to the input shaft of the automatic transmission. An electric machine that can be operated optionally as a motor or as a generator is arranged coaxially over the input shaft of the automatic transmission and the rotor of the electric machine is connected in a rotationally fixed manner directly to the input shaft of the automatic transmission. To supply oil to the automatic transmission an oil pump is provided, which can optionally be powered by actuating an associated clutch either by the driveshaft of the internal combustion engine or by an associated electric motor. In the second embodiment variant illustrated in FIG. 2 of the document the clutch for controlling the drive of the oil pump is arranged between the driveshaft of the internal combustion engine and a shaft section connected to the input element of the separator clutch for driving the oil pump.
Another hybrid drive train of this type with a planetary automatic transmission is known in two versions from DE 103 46 640 A1. In both versions of this known hybrid drive train, as illustrated in FIGS. 1 and 2 thereof, in each case an electric machine that can optionally be operated as a motor and as a generator is arranged coaxially over the input shaft of the automatic transmission and the rotor of the electric machine is in each case connected in a rotationally fixed manner directly to the input shaft of the automatic transmission. The driveshaft of the internal combustion engine, which is provided with a rotational vibration damper, can in each case be connected by a controllable separator clutch to the input shaft of the automatic transmission. In addition, a drive output element for an oil pump provided for supplying oil to the automatic transmission is in driving connection with the input shaft of the automatic transmission.
A further conventional hybrid drive train is described for example in DE 195 03 500 C2. The driveshaft of the internal combustion engine can be connected by a controllable separator clutch to the input shaft of the gearshift transmission. An electric machine that can be operated as a motor and a generator is arranged coaxially over the input shaft of the transmission, and the rotor of the electric machine is connected directly and in a rotationally fixed manner to the input shaft of the transmission. As a special feature of this known hybrid drive train the gearshift transmission does not comprise any reverse gearset for a reversing gear. Rather, to drive in reverse it is provided that the separator clutch is disengaged and the input shaft of the transmission is driven, with a forward gear engaged, by motor operation of the electric machine contrary to the rotation direction of the driveshaft of the internal combustion engine. In this operating condition the internal combustion engine can be switched off or run in the idling mode.
In these known hybrid drive trains it is disadvantageous that the speed of the electric machine connected to the input shaft in each case corresponds to that of the internal combustion engine, and accordingly, to produce sufficient drive power for purely electric driving operation the electric machine concerned must be relatively large and heavy. Furthermore, in boost-starting operation the increase of the engine torque of the internal combustion engine by the motor torque of the electric machine and hence the starting acceleration of the motor vehicle concerned and its climbing capacity from rest, particularly when starting uphill and when more heavily loaded, are relatively limited.
This disadvantage is at least partially overcome in a hybrid drive system described in DE 100 12 221 A1, which comprises a main drive train and an auxiliary drive train. In the main drive train, which corresponds to the hybrid drive train considered here, the electric machine concerned is arranged axis-parallel to the input shaft of the driving transmission, and the rotor of the electric machine is in driving connection with the input shaft of the driving transmission via an input transmission stage with a high gear ratio which can optionally be in the form of a spur gear pair or a belt-type transmission. The driving transmission is preferably in the form of a planetary automatic transmission, but can also be a manually or semi-automatically shifted variable-speed transmission.
By virtue of a high gear ratio of the input transmission stage, the speed of the electric machine is made slower and thus the torque of the electric machine acting upon the input shaft of the automatic transmission is correspondingly increased. This can be used to make the electric machine lower-powered and correspondingly smaller and lighter, or for electric-drive operation and boost operation, in particular when starting off uphill or with a heavier load, to provide a higher starting torque. The disadvantage of this known hybrid drive train, however, is it's large radial dimensions in the area of the electric machine and the input transmission stage, because of which fitting it into a motor vehicle as an alternative to a conventional drive train is not possible without major modifications of the vehicle chassis or vehicle body.
A further problem of the hybrid drive trains mentioned above is that the traction force is particularly weak during internal combustion engine operation, especially when starting and maneuvering. It is true that with appropriate design of the electric machine, during boost operation of the electric machine a relatively high starting torque can be produced at the input shaft of the automatic transmission. However, this is only possible when the energy accumulator for supplying the electric machine is sufficiently charged. On the other hand, if starting has to be carried out with the internal combustion engine alone and if an internal combustion engine of comparable torque power is used, due to the lack of additional torque from a torque converter only a substantially lower starting torque is now available, which is moreover delivered to the input shaft of the automatic transmission with slipping operation of the separator clutch that in this case acts as the starting clutch. As a result, compared with a conventional drive train the starting acceleration and climbing capacity on starting obtained are substantially lower. In addition, during internal combustion engine operation for starting and maneuvering there is a risk of overheating the separator clutch.